Blasting means



Feb. 28, 1956 c. o. DAVIS ETAL 3 BLASTING MEANS Filed July 20, 1950 4Sheets-Sheet 1 INVENTORS.

CLYDE 0. DAVIS, RICHARD C.GLOGAU, FRANK A. LOVING 6 JAMES P. swsoATTORNEYS Feb. 28, 1956 c. o. DAVIS ETAL 2,736,261

BLASTING MEANS Filed July 20, 1950 4 Sheets-Sheet 2 -O.6 -04 TIME INSECONDS PRESSUR E- THOUSANDS P.S.l. INVENTORS:

CLYDE 0. DA v/s, RICHARD c. GLOGAU, FRANK A.LOVIN6 &72 JATMEIS P.- SWEDW,Zj%%

ATTORNEYS Feb. 28, 1956 c. o. DAVIS ETAL 2,736,251

BLASTING MEANS Filed July 20, 1950 4 Sheets-Sheet 3 -2 TIME INMILLISECONDS RATE=L2X 10 nsllsec PRESSURE THOUSANDS P.S.|.

INVENTORS: CLYDE 0.0AV/S,

RICHARD c. GLOGAU, FRANK A. LOVING 8r JAMES SW D 452/6 W //.4.-.44

A TTORNEYS Feb. 28, 1956 c. o. DAVIS ETAL 2,736,261

BLASTING MEANS PRESSURE- THOUSANDS P.S.|.

CIIVVENTORS:

CLYDE 0. DA v/s, RICHARD c. GLOGAU, FRANK A.LOVING (:1 JAMES P swzoATTORNEYS limited States Patent 6 BLASTING MEANS Application July 20,1950, Serial No. 174,818 Claims. (Cl. 102-23) This invention relates toa new and improved blasting means, and particularly one which is safefor use in gassy coal mines.

A great number of different devices have been proposed for the mining ofcoal and other types of rocks where shattering is to be avoided.Probably the most common means have been the use of blasting explosivesof various types. Since these materials are explosive in themselvesparticular precautions must be taken in handling and storing them. Theyare subject to accidental initiation which is undesirable, and aremostly rapid and shattering in their effect, yielding a largerproportion of small particles than is ordinarily desired.

Another common means involves the rapid heating of compressed orliquified gases. This also is accom. panied by many disadvantagesincluding the difficulty of maintaining gases under high pressure incontainers for long periods of time, making performance uncertain andalso involving the danger of accidental initiation. These difficultiesare present whether carbon dioxide or air is involved.

In another method mixtures of solids have been confined inpressure-resistant tubes, but up to the present time this has involvedso many disadvantages that no satisfactory commercial operations havebeen achieved. Mixtures of this type include the Hydrox powders whichcontain for example an alkali metal nitrite and an antmonium salt. Theseare inherently extremely unstable and besides have all of thedisadvantages whichcharacterize electrically-fired, flame orspark-producing initiating means, since these are confined within thecontainers at all times after the initial loading.

Other mixtures of gas-generating solids which have been proposedcomprise ammonium nitrate and a catalyst or ammonium nitrate and a largeproportion of a fuel. Gas-generating cartridges of the prior artcontaining such mixtures are capable of violent explosion rather thanthe desired production of gas at a controlled rate. When an ammoniumnitrate-containing gas-generating composition fills the whole of apressure-resistant tubular 0ontainer, its decomposition is subject toincrease in pressure suflicient to burst the tube. When such a chargefills even one end only of the tube from wall to wall, it may burst thetube during decomposition. Charges introduced loosely into thepressure-resistant container Without means for keeping them in apredetermined location within the tube may, on handling, assume aposition within the tube which will result in their explosion ratherthan in their evolution of gas at a controlled rate. In addition,compositions having an excess of fuel or combustible may, upon reaction,eject hot, solid, unreacted material into the flammable atmosphere ofthe mine. Catalyzed ammonium nitrate compositions or ammonium nitratemixtures containing a large proportion of fuel, previously proposed, areordinarily ignited by flameor spark producing means fired by electriccurrent. They may thus become activatedaccidentally by stray currents,for example, or in error before the charge is in place in the 2,735,261Patented Feb. 28, 1956 borehole. Also, the charges must be prepared awayfrom the face, since the ignition devices are hazardous. Furthermore,the ignition devices proposed heretofore have been vigorous in theiraction. Their vigor is suflicient to hurl at least a part of thegas-generating charge against the end of the tube opposite the igniter,there to become packed solidly over the whole cross-sectional area ofthe tube... A further disadvantage of mixtures of the ammonium nitratetypes of the prior art is the fact that they yield toxic fumes asdecomposition products. Catalyzed ammonium nitrate compositionscontaining chromate or other chromium compounds as catalysts, forexample, cannot be used in coal mines because of the formation .ofnitrogen tetroxide upon their decomposition. Mixtures of ammoniumnitrate with a large amount of fuel contain an inadequate supply ofoxygen to oxidize them completely, and they thus develop toxic fumes oncombustion. Pressure-resistant tubes containing chemical compositions incontact with the closure disc require strong, chemically resistantdiscs. Ammonium nitrate mixtures containing a catalyst are relativelyexpensive because of the required addition of catalyst. Suchcompositions, and those containing a large proportion of fuel, are alsoexpensive to use because they require special handling and relativelyexpensive igniters.

An object of the present invention is to provide a safe device andmethod for the blasting of coal, 2. device and method that are free fromthe hazards of explosion, accidental initiation, ignition of flammablegases, evolution of toxic gases, and deterioration on storage. A furtherobject is to provide such a device and method which is highly effectivein yielding broken material in large pieces rather than in shatteredfragments. A still further object is to provide such a device and methodwhich are inexpensive, easy to use, and readily controllable. Additionalobjects will be disclosed as the invention is described more fullyhereinafter.

We have found that the foregoing objects are accomplished by heating,preferably by means of an electric resistance heater, a gas-generatingcomposition comprising ammonium nitrate, said gas-generating compositionbeing disposed axially within a pressure-resistant container having afrangible closure in such a way that the rate of pressure development ofthe gases evolved by the gas-generating composition within the containeris controlled to prevent the bursting of the pressure-resistantcontainer before the frangible closure is sheared by the gases underpressure, the gas-generating composition being desirably disposedaxially within the pressure-resistant container in such a way that at notime can the ammonium nitrate-containing composition pack the tube fromwall to wall at any point in the tube. We have found that thisarrangement may conveniently be accomplished by en closing thegas-generating composition within a thinwalled, substantiallycylindrical container that is smaller in diameter than the internaldiameter of the pressureresistant container. Preferably the diameter ofthe inner container will be not more than of the inner diameter of thepressure-resistant container, however this should not be taken aslimiting, it being necessary that the ammonium nitrate-containingcomposition should not completely jam the cross-section of the tube atany point so as to prevent the flow of gases to such an extent as torupture the tube. In other Words, the loading density of anycross-section of the charge-containing portion of the pressure-resistanttube must be low enough so that the charge will not serve to stopcompletely the flow of gases in the tube at that point. Futher detailsconcerning this loading density are set forth later on in thedescription of the invention.

It is also a desirable feature of the device that there shall be acontinued release of energy, that is a continued flow of gases, afterthe rupture of the frangible closure disc. To accomplish this result, itis necessary to select a charge having a predetermined rate of pressurerise. The limitations concerning this will be set forth in detail in thefollowing description.

It is also desirable that the device should perform in such a manner asto avoid ignition of any inflammable gases which might be present in thecoal mine. This is accomplished by limiting the fuel in the charge toprevent the occurrence of too much flame after the closure disc hasruptured. In fact, we prefer to so limit the fuel that no flame occursoutside of the pressure tube as would ignite inflammable gases. Suitableamounts of fuel for this purpose will appear from the examples in thefollowing description. It is also desirable to use a non-sparkingclosure disc in this connection.

The invention is further illustrated by the appended drawings, which arenot to be taken as limiting in any way. Referring generally to thedrawings, Figure l is a view in cross-section, of a pressure-resistanttube of the type known for the bringing down of coal and, containedtherein, a two-section gas-generating charge in a substantiallycylindrical container, said gas-generating charge being provided with anelectric resistance heater. Figure 2 is a similar view of another formof packaged gas-generating charge.

Figures 3, 4 and show piezo-gage records of the pressures obtained byfiring gas-generating charges such as are illustrated in Figures 1 and 2in pressure-resistant containers provided with frangible discs, such asare illustrated in Figure l. Pressures in thousands of pounds per squareinch are recorded on the ordinates of Figures 3, 4 and 5 and the time onthe abscissae, in seconds in Figure 3 and in milliseconds in Figures 4and 5. The curves thus represent the pressure as a function of the time,or the rate of pressure rise. Figure 4 shows the record of a chargewhich gives a rate of pressure rise of 1,200,000 pounds per square inchper second, and Figure 5 the record of a charge which gives a rate ofpressure rise of 3,500,000 pounds per square inch per second.

In detail, Figure 1 illustrates one arrangement for breaking coal inaccordance with our invention. In Figure l, 1 represents in longitudinalsection a tubular pressure-resistant container of steel. Containers ofthis general form are already in use for containing charges ofcompressed carbon dioxide. At one end of the tubular steel container arupture disc 2 of steel or other rigid material of such strength as tobe sheared at a predetermined pressure is held in place against aseating at the end of tube 1 by means of a hollow threaded cap 3, whichis provided with vent holes 4 for the escape of the gases released bythe shearing of the rupture disc and with an aperture 5 at the outer endfor facilitating removal of the sheared-off portion of the disc. At theopposite end of the steel tube 1 is a firing head 6 which comprises athreaded plug screwed in gas-tight manner against a shoulder at the endof tube 1 and which has an annular ridge 7 facing the interior of thetube. Between the firing head 6 and the shoulder of tube 1 is a seatingdevice 8 comprising a flanged plug of wood or other rigid nonconductingmaterial provided with two holes 9 and 10 for the passage of the ends ofa narrow strip of Nichrome ribbon 11. The end of the ribbon passingthrough hole 9 makes contact with electrode 12, which is insulated fromthe firing head 6 as it passes through a channel in the firing head toterminal 13, said channel being made gas-tight by any desired means (notshown). The end of the ribbon passing through hole 8 makes contact withannular ridge 7, and thus comes in metallic contact with firing head 6,to which a terminal 14 is attached. Terminals 13 and 14 are connected bytwo electric wires to a source of current. Attached to and closed byseating device 8 and extending longitudinally away from the firing headtoward the end of the tube closed by the rupture disc is thegas-generating charge. In this figure the gasgenerating charge is madeup in the following manner. A cylindrical cardboard container 15 whichhas as its base two vent holes 16 is attached to and closed by seatingdevice 8. The narrow strip of Nichrome ribbon 11 extends from electrode12 through hole 9 down through the cylinder 15, passes out of the baseof the cylinder through one of the holes 16, back into the cylinder 15through the second hole 16, and up through hole 10 into contact withridge 7. Around the Nichrome ribbon, cylinder 15 is filled with amixture of ammonium nitrate and starch 10% in an amount of approximately60 grams. To the base of vented cylinder 15, and, again, extending in alongitudinal direction away from the firing head, is attached asubstantially cylindrical bag 17 of a polymerized ethylene plasticsheeting containing approximately 265 grams of granular ammoniumnitrate.

The method of operation of the blasting device illustrated by Figure lis as follows. Closure disc 2 is inserted against the seating of tube 1and is held in place by cap 3, which is screwed in firmly. Seatingdevice 8 and the cardboard cylinder 15 and attached polythene bag 17containing the ammonium nitrate charges fastened to the seating device 8are inserted into the firing end of the tube and are held in place bythe flanged circumferential portion of seating device 8 which rests on ashoulder at the firing end of the tube. The firing head is screwed on,the tube is inserted in the borehole, terminals 13 and 14 are connectedby two electric wires to a source of current, and current of 5 to 15amperes and 24 to 40 volts is applied. The electric current heats theNichrome ribbon '11 to a temperature sufiiciently high to cause theammonium nitrate mixture in the cylinder 15 to begin to decomposelocally. The increase in temperature and pressure engendered herebyinitiates further reaction of the ammonium nitrate mixture, and thegases evolved escape through the vent holes 16 into the ammonium nitratein the polythene bag 17. The heat from these gases, the heat from thedecomposition of the ammonium nitrate mixture and additional heatsupplied by the heating element, and the pressure caused by the gaseousproducts of decomposition confined within the pressure-resistant tube 1all act to cause the reaction to proceed throughout the ammoniumnitrate. The pressure within the tube thus builds up gradually at first,and later at a higher rate; in approximately 10 to 25 seconds, the disc2 is sheared, and the gases escape through vent holes 4 to perform workagainst the material to be blasted.

Figure 2 shows another variation of a gas-generating charge for use in apressure-resistant container for blasting coal as illustrated inFigure 1. In Figure 2, instead of a charge in two sections, there is asingle charge contained in a substantially cylindrical container 18 offiame-resistant paper attached directly to seating device 8. The chargein container 18 comprises, for example, a mixture of granular ammoniumnitrate 88%, starch 10%, and sodium thiosulfate 2% in the amount of 350grams. Suspended in this charge through holes 9 and 10 is a loop ofNichrome ribbon 11. The method of operation of the charge of Figure 2 isessentially the same as that of the two-section charge of Figure l, theupper part of the container 18 of Figure 2 being self-vented as thecharge decomposes at high temperature.

Figures 3, 4 and 5 show piezo-gage records of pressures obtainedimmediately before and after the shearing of the frangible disc inpressure-resistant tubes when gas-generating charges in accordance withour invention are fired therein. Figure 3 shows the pressure phenomenain the 1.4 seconds before maximum pressure is reached. The pressure inthe pressure-resistant tube builds up very slowly in the first 20seconds, approximately, after initial application of the firing currentand this phase is not shown. The pressure rise becomes rapid, beginningabout the last second before the maximum is reached as shown in Figure3, and still more rapid in the last tenth of a second precedingthemaximum. 1 The pressure phenomena occurring during the last 12milliseconds before the attainment of maximum pressure are illustratedmore clearly in Figures 4 and 5, where the time scale is in millisecondsrather than in seconds. Figure 4 shows the pressure record of a chargeof such dimensions and distribution as to give a rate of pressure riseof 1,200,000 pounds per square inch per second. It will be seen that thedisc was sheared about 1 millisecond before the maximum pressure wasreached (this point is indicated on the piez'o gage by mechanicalvibration of the instrument following the rupture of the disc). Figure 5shows the piezo-gage record made by a charge which gives a rate ofpressure rise of 3,500,000 pounds per square inch per second. It will benoted that the slope of the curve is steeper during the same period oftime (12 milliseconds) before the maximum pressure is reached, and thatthe difference in pressure between the point of rupture and the maximumis greater than in Figure 4.

EXAMPLE 1 Into a pressure-resistant container of the type illustrated inFigure 1 having a capacity of 130 cubic inches and an internal diameterof 1.8 inches and provided with a rupture disc of a strength towithstand a static pressure of 9000 lb./sq. in. was introduced at thefiring end a charge made up of (1) 125 grams of a mixture of granularammonium nitrate 90% and granular starch in a cardboard tube providedwith three vent holes along the side and with a heating elementconsisting of Nichrome ribbon passing in a loop from one electrode atthe firing end of the pressure-resistant tube to the bottom of thecardboard tube and back to the second electrode, and (2) a further 100grams of the same mixture in a cylindrical bag of polythene plasticfastened to the free end of the cardboard tube. The diameters of thecardboard tube and of the cylindrical polythene bag were substantiallythe same, and were approximately 34% less than the internal diameter ofthe cylinder. The loading density of the ammonium nitrate in thepressure-resistant tube was 0.10 gram/ cubic centimeter,,and that of thetotal charge was 0.11 gram/ cubic centimeter. The cross-sectionalloading density of the ammonium nitrate in the portion of thepressureresistant tube containing the charge was 0.35 gram/cubiccentimeter. The firing head was closed and a current of 8 amperes at 36volts was applied for approximately 25 seconds. The disc ruptured 35seconds after the initial applicationof current.

EXAMPLE 2 A charge formed in the manner of Example 1 but containing (1)60 grams of a mixture of ammonium nitrate 84%, granular starch 10% andNazSzOaSHzO 6%, the latter as an anti-segregating agent, and (2) 265grams of a mixture of ammonium nitrate 90% and granular starch 10% wasinserted in a pressure-resistant container of the type illustrated inFigure 1 having a capacity of 130 cubic inches and an internal diameterof 1.8 inches, and a static rupture disc strength of about 9000 poundsper square inch, the loading density of the ammonium nitrate in thepressure-resistant container being 0.14 gram per cubic centimeter, thatof the total charge being 0.16 gram per cubic centimeter and thecross-sectional loading density of the ammonium nitrate in the portionof the pressureresistant tube containing the charge being 0.35 gram percubic centimeter. The charge was fired by the application of a currentof 8 amperes at 39.6 volts for approximately 10 seconds. The rupturedisc sheared 17 seconds after application of current was begun.

EXAMPLE 3 A shot similar to the one described in Example 2 was made,except that 260 grams of granular ammonium nitrate alone was containedin the part of the charge in the polythene bag. The loading density ofthe am- EXAMPLE 4 A mixture of 88% of granular ammonium nitrate, 10% ofgranular starch, and 2% of Na2S2O3.5H2O was loaded into aflame-resistant paper tube 1% inches in diameter and 24 inches longwhich was provided with a loop of Nichrome wire to serve as a heatingelement. The gas-generating charge was introduced into apressureresistant container of the type'illustrated in Figure 1, whichwas provided with a steel rupture disc (capable of withstanding a staticpressure of 18,000 to 20,000 pounds per square inch). The loadingdensity of the ammonium nitrate in the pressure-resistant tube was 0.15gram per cubic centimeter and that of the total charge was 0.17 gram percubic centimeter. The cross-sectional loading density of the ammoniumnitrate in the portion of the pressure-resistant tube containing thecharge was 0.35 gram per cubic centimeter. The firing head was screwedon, contact with a 24-volt battery was made, and a current of 13 amperesfrom this source was applied for 18 seconds, at which time the heatingwire burned through (as recorded by an ammeter in the firing circuit),and the circuit was thus opened. One second later the disc sheared.

EXAMPLE 5 A current of 13 amperes and 24 volts was applied to anassembly like that of Example 4 for a period of 10 seconds, after whichthe current was cut off by the opening of a control switch. The disc didnot rupture. When the gas-generating charge in its package was removedfrom the pressure-resistant tube, broken apart, and examined, it wasfound that the ammonium nitrate-containing gas-generating charge hadmelted somewhat in the vicinity of the Nichrome ribbon heater and hadresolidified. The initial heating had thus not proceeded far enough tocause the reaction to be self-sustaining. Another charge of the sametype was subjected to the same treatment, except that it was not brokenapart for examination, but was inspected only externally. This samecharge was replaced in the pressure-resistant container and suppliedwith a firing current of 13 amperes at 24 volts. The rupture disc wassheared in the normal manner 22 seconds after the initial application ofcurrent.

EXAMPLE 6 A current of 14 amperes at 24 volts was applied to an assemblylike that of Example 4 for a period of 15 seconds. The current was thencut off, and the disc ruptured 5 seconds later.

EXAMPLE 7 Into a pressure-resistant container of the type illustrated inFigure 1 having a capacity of cubic inches and an internal diameter of1.8 inches and provided with a steel rupture disc capable ofwithstanding a static pressure of 18,000-20,000 pounds per square inchwas introduced 350 grams of a mixture of granular ammonium nitrate 96%and granular starch 4% in a cylindrical polythene bag having an externaldiameter of 1%; inches, the bag containing the mixture being provided atthe firing end with a heating device comprising an electrically firingsquib placed in such a way in the bag as to be surrounded by theammonium nitrate mixture. The loading density of the ammonium nitrate inthe pressure-resistant tube was 0.16 gram per cubic centimeter and thatof the total charge was 0.17 gram per cubic centimeter. The

7 cross-sectional loading density of the ammonium nitrate in the portionof the pressure-resistant tube containing the charge was 0.41 gram percubic centimeter. The firing head was closed and current was applied.The rupture disc was sheared in less than one second, without damage tothe pressure-resistant tube.

When, however, the same quantity of the ammonium nitrate/fuel mixturewas poured loosely into a similar tubular container having a similarsteel rupture disc, and around a similar squib at the firing end in sucha way that the granular composition filled the space around the heaterfrom wall to wall of the pressure-resistant tube and had a loadingdensity of the ammonium nitrate in the pressure-resistant container of0.16 gram per cubic centimeter, that of the total charge being 0.17 gramper cubic centimeter, and the cross-sectional loading density of theammonium nitrate in the portion of the pressureresistant tube containingthe charge being 1.0 gram per cubic centimeter, the tube itself wasruptured in less than one second after application of current.

EXAMPLE 8 A copper tube 3 inches long and /2 inch in diameter containinga centrally located longitudinal copper electrode out of contact withthe copper tube was filled, between tube wall and electrode, with aconductive mixture containing ammonium nitrate 99% and graphite 1%, themixture being pressed into the tube. A cylindrical polythene bag 1%inches in external diameter and containing 300 grams of a mixture ofammonium nitrate 88% and granular starch 12% was arranged about thecopper tube. The assembly was introduced into a pressure-resistantcontainer of the type illustrated in Figure 1 having a volume of 130cubic inches, an internal diameter of 1.8 inches, and a rupture disc ofa strength to withstand a static pressure of 9000 pounds per squareinch. The loading density of the ammonium nitrate in thepressure-resistant tube was 0.13 gram per cubic centimeter and that ofthe total charge was 0.15 gram per cubic centimeter. The maximumcross-sectional loading density of the ammonium nitrate in the portionof the pressure-resistant tube containing the charge was 0.4 gram percubic centimeter. A direct current of 33 volts was applied. The sheardisc ruptured after two minutes.

EXAMPLE 9 A gas-generating charge comprising 420 grams of a mixture ofammonium nitrate 96% and granular starch 4% was loaded into a tin can 1/8 inches in diameter, which contained a heating ribbon of Nichrome. Thegasgenerating charge was introduced into a pressure-resistant containerof the type illustrated in Figure l which was provided with a fiberrupture disc (capable of withstanding a static pressure of about 9000pounds per square inch) and which had a capacity of 130 cubic inches andan internal diameter of 1.8 inches. The loading density of the ammoniumnitrate in the pressure-resistant tube was 0.20 gram per cubiccentimeter and that of the total charge was 0.21 gram per cubiccentimeter. The crosssectional loading density of the ammonium nitratein the portion of the pressure-resistant tube containing the charge was0.38 gram per cubic centimeter. The firing head was screwed on, contactwith a source of electricity was made, and a current of 15 amperes at 24volts was applied; the fiber disc sheared after 94 seconds withoutdamage to the tube.

In another firing, however, 100 grams of a mixture containing 90% ofammonium nitrate and 10% of starch was placed in a similar can providedwith a Nichrome heating element and was introduced into the firing endof the tube. From the opposite end of the tube 300 grams of the samemixture was poured around the outside of the can, filling the tube fromwall to wall at the firing end. The loading density of the ammoniumnitrate in the pressure-resistant tube was 0.18 gram per cubiccentimeter and that of the total charge was 0.20 gram per cubiccentimeter. The cross-sectional loading density of the ammonium nitratein the portion of the pressure-resistant tube containing the charge was1.0 gram per cubic centimeter. A plastic rupture disc capable ofwithstanding a static pressure of about 9000 pounds per square inch wasinserted in the opposite end of the tube. An electric current of 8amperes at 38 volts was applied by way of the electrodes at the firingend, and, in this case, the tube itself blew apart after 50 seconds.

EXAMPLE 10 A mixture of of granular ammonium nitrate and 10% of granularstarch in the amount of 300 grams was loaded into a cylindricalpolythene bag 1% inches in diameter, and 25 grams of smokeless shotgunpowder around an electric bridge wire assembly provided with a bead ofignition material on the bridge wire was inserted for ignition in theend of the bag which was attached to the seating device. Thisgas-generating charge was inserted in a pressure-resistant container ofthe type illustrated in Figure 1 having a capacity of cubic inches andan internal diameter of 1.8 inches and being provided with a rupturedisc of sufiicient strength to withstand a static pressure of 20,000pounds per square inch, the loading density of the ammonium nitrate inthe pressureresistant container being 0.13 gram per cubic centimeter,that of the total charge being 0.15 gram per cubic centimeter and thecross-sectional loading density of the ammonium nitrate in the portionof the pressure-resistant tube containing the charge being 0.35 gram percubic centimeter, and fired. The charge ruptured the disc inapproximately one second without damage to the tube. on the other hand,when the same quantity of a similar mixture, poured loosely into thefiring end of a tube of the type illustrated in Figure 1, around asimilar ignition means, the loading density of the ammonium nitrate inthe pressure-resistant container being 0.13 gram per cubic centimeter,that of the total charge being 0.15 gram per cubic centimeter, and thecross-sectional loading density of the ammonium nitrate in the portionof the pressureresistant tube containing the charge being 1.0 gram percubic centimeter, was fired, the tube itself was ruptured in about onesecond.

In the preceding examples it will be noted that in each case the loadingdensity of the ammonium nitrate is less than that of the total charge.Thus, when we speak of the loading density of the ammonium nitrate, wemean the loading density of the ammonium nitrate content of the charge,and not that of the whole charge.

It will be seen from the foregoing examples that a very effective meansof releasing gas under pressure for breaking coal or other material isprovided by the heating of a gas-generating composition comprisingammonium nitrate in a pressure-resistant tube having a frangible closureat one end, so long as the ammonium nitrate-containing charge ismaintained in a position in the tube permitting a channel for the escapeof the gases produced by the decomposition of the ammonium nitratecharge as soon as they are formed. When the gas-generating charge fillsthe tube throughout the entire crosssectional area of thepressure-resistant tube at any point, as is pointed out in Examples 7,9, and 10, there is immediate danger of rupture of thepressure-resistant tube before the rupture disc is sheared.

It has been found by measuring, by means of strain gages andpiezo-electric gages, the pressure engendered by the decomposition ofammonium nitrate-containing gas-generating compositions inpressure-resistant tubes provided with frangible closures, that thedynamic pressure at which the rupture discs shear may be higher than thestatic (hydraulic) pressure that the discs are capable of withstanding.Thus, a steel rupture disc capable of 9 withstanding a static pressureof 8,000-10,000 pounds per square inch may be ruptured at a dynamicpressure of as high as 13,000 pounds per square inch. Furthermore, ithas also been found that the peak pressure attained by the decompositionof the ammonium nitratecontaining gas-generating compositions givingsatisfactory performance occurs not at the instant of rupture of thedisc, as would be expected, but at a'measurable interval (of less than lmillisecond) after rupture of thedisc. This may be seen clearly byreference to the curves of Figures 4 and 5. The difference between thepressure at rupture and the subsequent maximum pressure is the greater,the higher the rate of pressure deyelopment at the instant of rupture.This rate of pressure development increases with increase in chargediameter, and, when the gas-generating charge fills the tube completely,the rate is so high that the interval between attainment of the rupturepressure and the peak pressure is approximately zero, and the tube mayrupture simultaneously with the shearing of the disc, or even before. 1The relationships described in the foregoing will be understood moreclearly by reference to Table I, wherein rupture pressures, peakpressures, and rates of pressure development are given for series ofshots made with gas-generating compositions of various diameters insteel pressure-resistant tubes capable of withstanding a static pressureof about 35,000 pounds per square inch and having steel rupture discscapable of withstanding a static,pr.essure of 8,000 to 10,000 pounds persquare inch. ,7 The tubes had an internal diameter of 1.80 inchesand-acapacity of 130 cubic inches. The charges were prepared as shown inFigure 2 and were all of the same weight and composition: 250 grams of amixture of granular ammonium nitrate 88%, starch 10%, and so diumthiosulfate 2%. With the smaller charge diameters, it is evident, thecharges were longer, since the weights of gas-generating compositionwere the same. The loading density of the ammonium nitrate in thepressure-resistant tube was 0.15 gram per cubic centimeter .and that ofthe total charge was 0.17 gram per cubic centimeter in each case.

T able I'. D.ynamic pressure data with 8,00010,000 pound rupture disc Hp v Diameters of charges '1 in. 1% in. 1% in. 1% in.

Disc rupture pressure,

lb./sq. in., Piezo gage 8, 800 8, 600 10, 100 13, 400 Peak pressure, lb/s in.:

' Strain gage 11, 700 14, 100 15, 600 21, 800 Pieio gage. 10, 100 10,100 13, 500 16, 700 Rate ot press. rise, lb./ sg. in./sec.

Strain gage 1, 100, 000 1, 400, 000 3, 000, 000 6, 900, 000 1 Piezo gage1, 000, 000 1, 000, 000 3, 900, 000 7, 500, 000

Table II.'-Cr0ss-s ecti0nal densities of the ammonium -rzitrate in thecharges used in Table I at various charge diameters Diameter of charge,in inches"; 1

Cross-sectional loading density of NHiNO: in the portion of thepressure-resistant tube containing the .charge 0.27 0.35

fvery uncertain in its results.

ameter for a packaged charge which can be loaded, although withdifiiculty, in a tube 1.8 inches in diameter), the rate of pressure riseis approximately twice as high as that for a charge 1% inches indiameter. For convenience in handling, and, more important, to providean adequate factor of safety, since any one charge could have pressuresappreciably higher than the average shown in the table, we prefer to usecharges which will not exceed a rate of pressure rise of 4,000,000pounds per square inch per second, and it is even more desirable to usecharges which will not exceed a rate of pressure rise of 3,000,000pounds per square inch per second.

The ratio of ammonium nitrate by weight in the gasgenerating compositionto the volume of the pressureresistant tube, or the loading density ofthe ammonium nitrate in the pressure-resistant tube, is, in the examplescited, of the order of 0.2 gram per cubic centimeter. The loadingdensity of any cross-section of the chargecontaining portion of thepressure-resistant tube is approximately 0.4 gram per cubic centimeterfor those charges giving satisfactory performance. When gasgeneratingcharges containing ammonium nitrate are prepared of such diameter andweight that the loading density of the ammonium nitrate in thepressure-resistant container does not exceed 0.3 gram per cubiccentimeter and the cross-sectional loading density of the ammoniumnitrate, as distinct from the loading density of the total charge, doesnot exceed 0.5 gram per cubic centimeter at any point, the rate ofpressure development will be sufficiently low to avoid the possibilityof breaking pressure-resistant tubes such as are suitable for coalmining, as may be seen further from the cross-sectional densities givenin Table II, taking into account an adequate factor of safety.

The thermal decomposition of ammonium nitrate or of ammoniumnitrate-fuel mixtures has heretofore been It will be seen from thecurves of Figures 3, 4 and 5 that in the thermal decomposition ofammonium nitrate a building up of pressure proceeds gradually uponcontinued application of heat.

This pressure build-up is extremely slow at the start.

It has been established that the thermal decomposition of ammoniumnitrate is sensitive to pressure, i. e., the rate of decomposition ofammonium nitrate, and hence the rate of rise in pressure engenderedthereby increases with increase in the pressure on the ammonium nitrateundergoing decomposition. This rate of pressure rise has been found byus to depend on the concentration (or crosssectional density) ofammonium nitrate in that part of the pressure-resistant containercontaining ammonium nitrate, when ammonium nitrate is heated underpressure in a pressure-resistant container. In our invention, gaspressure is permitted to develop as a result of the confinement of ourgas-generating charge within the pressureresistant container, but it isprevented from arriving at dangerous levels locally by provision ofchannels for the escape of gases. Our gas-generating charge, which isheated locally, stays in its predetermined position during the slowphase of the reaction, and the provision of channels around or throughthe charge permits the gases to escape from the vicinity of the charge,and thus prevents local pressure rise sufficient to burst the tubebefore the rapid phase of the reaction begins. The charge is preventedfrom reaching an overall pressure that will rupture the tube by being soarranged geometrically, as described in the foregoing, that the rupturedisc breaks 11 before the peak pressure is achieved, and a hazardousrate of pressure rise of over 4,000,000 pounds per square inch persecond is not attained. Thus a rapid rate of pressure rise leadingtoward explosion is avoided under conditions which would otherwise tendto produce explosion rather than a controlled generation of gas.

While we have shown an annular channel around the ammoniumnitrate-containing gas-generating charge to provide for the escape ofgases formed on decomposition of the charge, it may also be desirable toprovide longitudinal channels within the charge for the purpose ofperforming the same function, said channels likewise reducing thecross-sectional charging density of the ammonium nitrate in thepressure-resistant tube. The charge may be in two sections, the firstsection being contiguous to the heating device and the second sectionattached thereto or in a separate package at some other location withinthe pressure-resistant tube.

The substantially cylindrical container in which our gas-generatingcharge comprising ammonium nitrate is packaged may be of any suitablethin material such as plastic sheeting, paper, light-weight cardboard,metal foil, or the like. It is necessary only that the material havesegregating agents such as sodium thiosulfate in ammonium nitrate/ fuelmixtures.

It has been found that ammonium nitrate-containing gas-generatingcharges of the foregoing compositions, when fired enclosed inthin-walled, substantially cylindrical containers inserted inpressure-resistant tubes of larger internal diameter than the diameterof the thin-walled container, the loading density of the ammoniumnitrate in the pressure-resistant container being less than 0.3 gram percubic centimeter and the cross-sectional density of the ammonium nitratein the pressure-resistant container being less than 0.5 gram per cubiccentimeter yield substantially less than the amounts of toxic gasespermitted in coal mines. The gases which issue from thepressure-resistant tube upon rupture of the frangible disc consistessentially of nitrogen, carbon dioxide, and water and contain onlyexceedingly small amounts of toxic gases, as may be seen from Table III,wherein are recorded the amounts of noxious gases formed by variousammonium nitrate-containing compositions fired in a pressurercsistantcontainer in accordance with our invention. These firings were made in aclosed chamber in order that the gases might be collected and analyzed.

Table III Gas-Generating Charge Noxious gases in percent of total gasesg fg g gggggg ilfi' g Total All N115! Amount (g.) Composition N0 N0: 00H18 Nagisgtsls Gases a g 8 2? 91.3% NH4NO3 130 7.3% Starch 0. 0 1- 10 1. 7 324 O. 9 5. 4

1.4% N212SzOa.5H20- 88- 3% NH4NO3 130 9.8% Starch 0- 0 1- 8 0- 4 2. 2358 0 7. g

1.9% N32520:; 51120- 88.3% NHlNO; 100 9.8% Starch Q 0 7 0 5- 7 355 O 20.2

1.9% NazSzOs.5Hz0- 87.9% NH4NO3 110 9.8% Starch 0- 0 6- 5 0- 3 6- 8 3550 24. O

, 2.3% N82S203.5H2

The U. S. Bureau of Mines permits 158 liters of noxious gases per 1.5pounds of a permissible powder and 5 liters o 1 total NO+NO2 per 1.5pounds of powder.

suflicient thickness and rigidity to keep the charge in place. It isdesirable that the packaging material be nonflammable or be madenonfiammable by appropriate coating or other treatment. It is likewisedesirable that the packaging material be moisture-proof, inasmuch asammonium nitrate is a hygroscopic material. The part of the containeraround the portion of the gas-generating charge where heat is appliedmay, if desired, be of somewhat thicker material than the remainder ofthe container. In that event, it may be desirable to provide vent holesfor the gases in the part of the container which is made of the thickermaterial, either in the side walls of the container or in the base ofthe first section of a two-section charge as illustrated in Figure 1.

Suitable compositions for use in our packaged gasgcnerating compositionsare ammonium nitrate alone, desirably of relatively coarse granulation,or ammonium nitrate with a small amount of a fuel, such as, for example,starch, wood pulp, petrolatum, engine oil, calcium stearate, orgraphite. It is preferable that the fuel be added in amounts such thatthe mixtures and packaging materials therefor contain no less than theamount of oxygen necessary for complete combustion of the mixture andits container. Thus, even with organic fuels containing a large amountof oxygen, the amount of fuel is not likely to exceed approximately 12%by weight of the gas-generating composition. Cooling salts such assodium chloride, borax, metallic carbonates, and the like, may also beadded, obviously only in quantities which are consistent with adequatefunctioning of the composition in question. It may also be desirable toinclude anti- While it is possible to heat charges of the type describedby means of heating devices which produce flame, such as are describedin Example 7 and 10, we prefer to use nonflame-producing heatingelements, for example, electric resistance heaters. A resistance heaterof the type illustrated in Figures 1 and 2 comprising a ribbon ofNichrome is particularly suitable. With such an electric resistanceheater, only current of moderate amperage and voltage need be appliedfor a time of several seconds. Heaters of Nichrome ribbon are alsoinexpensive, and, consequently, expendable. Moreover, a Nichrome ribbonheater is capable of supplying adequate heat to initiate thedecomposition of the ammonium nitrate-containing charge and ofcontinuing to supply heat for a time thereafter, until the ribbon hasburnt through. After the ribbon has burnt through, and the instant ofits burning through can be detected by means of an ammeter in the firingcircuit, the electric current may be cut off. Thus, at the time of thebursting of the rupture disc, no live wires need be present, which isadvantageous in maintaining the high degree of safety of thegas-generating charge. Another suitable form of electric resistanceheater is the copper tube assembly described in Example 8. In thisheater, the current passes directly through the graphite-containingammonium nitrate between the copper electrodes. It is likewise possibleto construct a heating element, such as a copper coil, as a permanent,reusable part of the pressure-resistant tube, which element may be incontact with the gas-generating charge package, without, however,interfering with the venting" channels therein. The heater may be placedat the firing 13 head end of the gas-generating charge or at anyconvenient location along the charge.

The frangible closure discs may be made of any desired material of anysuitable strength consistent with the release of gases at a pressureadequate for the breaking of coal or other hard material. While discs ofsteel may be used, we find it also advantageous to use discs of anonmetallic material such as fiber or plastic, since such discs arenonsparking when the portion sheared by the gas pressure strikes theventing cap. It is possible to use discs of such material as fiber orplastic with our package gasgenerating charges because the rupture discin our blasting assembly is not required to retain gases under pressureprior to use, nor is it in direct contact with any chemically reactivematerial. Closure discs of nonsparking metals such as brass may also beused with advantage.

While our invention has been illustrated by means of pressure-resistanttubes of the size and strength which have been in use for the bringingdown of coal, it will readily be understood that tubes of larger orsmaller diameter or length or of greater or lesser strength may be used,and that the amount and composition of the gasgenerating charge wouldthen be adjusted accordingly, and, consequently, the diameter and lengthof the package. Venting-space, density, and rate-of-pressure-riserelationships, however, would need to be maintained as set forth hereinin order to avoid the probability of rupture of the tube.

It will be seen from the foregoing description that assemblies whereinnonexplosive gas-generating compositions comprising ammonium nitrate areprepared in accordance with our invention afford an excellent means forbreaking materials such as coal in large lump form because of the slow,heaving action produced by the release of gases under pressure, themeans having the advantages of a high degree of safety, ease ofoperation, and economy.

The most important safety feature resides in the fact that thegas-generating charge is so arranged that the danger of its developinglocal high pressure sutficient to burst the tube when activated isavoided. Furthermore, the gas-generating charges of our invention yieldsmaller amounts of toxic gases than those permissible in coal mines. Inaddition, the charges are not hazardous to handle, since they cannot bedecomposed rapidly without the application of high temperatures andpressures and consequently are not subject to explosion by shock,impact, friction and the like. With the preferred type of heater,accidental activation of the heater could not cause decomposition of thewhole charge, and no flames or sparks are produced by the heatingelement; current greater than the stray currents likely to beencountered in mines must be applied not momentarily, but for anappreciable length of time to cause the device to function. When aheater of the Nichrome ribbon type is used, the ribbon is burned throughseveral seconds before the rupture of the disc; the current may thus becut off, leaving no live Wires to cause a spark at the time of shearingof the rupture disc. Other safety advantages are obtained because of thefact that nothing in the chargecan react of itself, nor leak out, andbecause of the fact that the reaction is so rapid that the tube is notheated through before discharge takes place (consequently, the tube isrelatively cool after discharge and can be handled immediately).

Ease of operation results from the fact that the charges may be preparedin convenient packets of proper size and amount of charge, which packetscan be inserted in the pressure-resistant tubes in the mine. Thepressure-resistant tubes thus do not have to be carried in and out ofthe mine for loading.

The blasting assemblies described are economical because commercialammonium nitrate, suitable packaging for our gas-generatingcompositions, and the preferred form of heater element, are all cheap.In addition, preparation of the charges is extremely simple, and thecharges are stable on storage. Moreover, the gaseous products ofdecomposition are not corrosive to the pressureresistant tubes. Thislack of corrosiveness of the decomposition products results in long lifeof the tubes, which may be reused repeatedly.

The invention has been described at length in the foregoing, but it willbe understood that many variations in details of charge compositions,assembly, and design of parts may be introduced without departure fromthe scope thereof. We intend, therefore, to be limited only by thefollowing claims.

We claim:

1. A blasting device especially adapted to coal mining comprising apressure-resistant tube provided with a rupture disc and a heatingmeans, and in contact with said heating means a gas-generatingcomposition comprising a major proportion of ammonium nitrate, saidgas-generating composition being so distributed along the axis of saidpressureresistant tube that the loading density of the ammonium nitratethroughout the tube is from 0.1 to 0.3 gram per cubic centimeter, and atany cross-section of the tube is maintained below 0.5 gram per cubiccentimeter, the solid-free space within said tube being occupied bygaseous material.

2. A blasting device especially adapted to coal mining comprising apressure-resistant blasting tube provided with a rupturable closuredisc, and containing a gas-generating composition comprising a majorproportion of ammonium nitrate, said gas generating composition beingenclosed in a cylindrical package of smaller diameter than the internaldiameter of the pressure-resistant tube, the diameter of the packagebeing not more than 75% of the internal diameter of saidpressure-resistant tube, the loading density of the ammonium nitratethroughout the tube being from 0.1 to 0.3 gram per cubic centimeter andat any crosssection of the tube being maintained below 0.5 gram percubic centimeter, the solid-free space Within said tube being occupiedby gaseous air, and heating means'for initiating said composition.

3. A blasting device especially adapted to coal mining of the typeincluding a pressure-resistant tube with a rupturable disc closuremeans, an inner substantially cylindrical container of thin material ofsmaller diameter than the internal diameter of said pressure-resistanttube being disposed substantially along the axis of said tube, saidinner container being charged with a gas-generating compositioncomprising a major proportion of ammonium nitrate and a fuel so disposedtherein that the loading density of the ammonium nitrate in thepressure-resistant tube is from 0.1 to 0.3 gram per cubic centimeter andat any cross-section of the pressure-resistant tube is less than 0.5gram per cubic centimeter, whereby a substantially annular space isprovided between the inner container and the internal wall of thepressure-resistant tube, said solid-free space being occupied by gaseousair, and an electric resistance heater disposed in contact with a minorproportion of said gas-generating composition.

4. A blasting device especially adapted to coal mining comprising apressure-resistant tube provided with a rupture disc and heating means,and in contact with said heating means a gas-generating compositioncomprising a major proportion of ammonium nitrate, the loading densityof said ammonium nitrate throughout the tube being from 0.1 to 0.3 gramper cubic centimeter and at any cross-section of the tube beingmaintained below 0.5 gram per cubic centimeter, the solid-free spacewithin said tube being occupied by gaseous air, said blasting devicebeing characterized by a rate of pressure rise as confined within saidtube not exceeding 4,000,000 pounds per square inch per second.

5. A method of blasting comprising heating in a pressure-resistant tubehaving a frangible closure at one end a gas-generating compositioncomprising a major proportion of ammonium nitrate, said gas-generatingcomposition being distributed along the axis of said pressureresistanttube in such amount that the loading density References Cited in thefile of this patent of the ammonium nitrate throughout thepressure-resistant UNITED STATES PATENTS tube is 0.1 to 0.3 gram percubic centimeter and at any cross-section of the pressure-resistant tubeis less than 1,610,274 Ferrell et a1 1926 0.5 gram per cubic centimeter,the solid free space being 5 1,705,248 Hart 1929 occupied by gaseounateriaL Lubelsky 1 1,950,038 Scott Mar. 6, 1934 2,463,709 McFarlandMar. 8, 1949

0.5 GRAM PER CUBIC CENTIMETER, THE SOLID FREE SPACE BEING OCCUPIED BYGASEOUS MATERIAL
 5. A METHOD OF BLASTING COMPRISING HEATING IN APRESSURE-RESISTANT TUBE HAVING A FRANGIBLE CLOSURE AT ONE END AGAS-GENERATING COMPOSITION COMPRISING A MAJOR PROPORTION OF AMMONIUMNITRATE, SAID GAS-GENERATING COMPOSITION BEING DISTRIBUTED ALONG THEAXIS OF SAID PRESSURERESISTANT TUBE IN SUCH AMOUNT THAT THE LOADINGDENSITY OF THE AMMONIUM NITRATE THROUGHOUT THE PRESSURE-RESISTANT TUBEIS 0.1 TO 0.3 GRAM PER CUBIC CENTIMETER AND AT ANY CROSS-SECTION OF THEPRESSURE-RESISTANT TUBE IS LESS THAN